Reed relay having a low thermal emf



July 15, 1969 J, c. BECKER ET AL 3,456,216

REED RELAY HAVING A LOW THERMAL EMF Filed Feb. 8, 1967 2 Sheets-Sheet 1 INVENTORS JOHN c. BECKER ALBERT BENJAMINSON JERRY A. BLANZ HENRY E. KARRER ROBERT J. MOFF'AT ATTORNEY July 15, 1969 c, BECKER ET AL 3,456,216

REED RELAY HAVING A LOW THERMAL EMF Filed Feb. 8,1967 2 Sheets-Sheet 2 igure ,4 M O z 22, 462 7 w no INVENTORS JOHN c. BECKER ALBERT BENJAMINSON JERRY A. BLANZ HENRY E. KARRER ROBERT J. MOFFAT BY Maw ATTORNEY United States Patent U.S. Cl. 335-151 9 Claims ABSTRACT OF THE DISCLOSURE A reed relay mounted in an isothermal enclosure and having a hollow thermally-conductive bobbin within which at least one reed switch is supported and on an outer surface of which a reed switch drive coil is wound so that heat is substantially transferred away from the drive coil to prevent it from contributing to a temperature dihierential between the ends of the reed switch, the ends of the reed switch being substantially thermally shorted together so as to reduce the temperature differential between the ends of the reed switch and being connected to the end terminals of the reed relay by thermally-resistive electrical conductors so that the thermally nnbalancing effects of external temperature gradients are substantially reduced.

Background of the invention This invention relates to a reed relay in which the temperature ditterential that can be created between the ends of the reed switch portion of the relay is minimized so as to substantially reduce the thermal EMF developed across this reed switch portion.

A reed relay comprises at least one reed switcha simple on-otf switch that is activated by an externally applied magnetic fieldplaced within a drive coil through which a D0. current is passed to generate a magnetic field parallel to the axis of the drive coil as required for closure of the reed switch. The reed switch comprises two flexible metal reeds hermetically sealed and supported in a glass enclosure so that the reeds are mechanically held apart except in the presence of a magnetic field. These reeds consist of an iron-nickel alloy which is selected because of its magnetic properties and because it has suitable properties for a glass-to-metal seal. Extensions of the iron-nickel reeds are brought out through the glass enclosure and form the ends of the reed switch. Two thermoelectrically-active junctions are formed when copper wires from an external circuit are connected to the iron-nickel ends of the reed switch. A temperature differential created across these junctions by heat dissipated in the drive coil, convective air currents, external heat generators, and other causes of ambient temperature gradients produces a thermal EMF due to the thermoelectric efiect. For example, typical reed switches produce thermal EMFs ranging to seventy-five microvolts and higher during steady state operation. Thermal EMFs of this magnitude cause serious problems in applications requiring the switching of microvolt signals.

Summary of the invention In is the principal object of this invention to provide a reed relay having a low thermal EMF on the order of less than a microvolt during steady state operation.

This object is accomplished according to the illustrated embodiment of this invention by physically separating the reed switch portion of the relay from the reed switch drive coil with a continuous thermally-conductive body so as to transfer heat away from the drive coil and prevent it from contributing to a temperature differential between the ends of the reed switch portion. The two thermoelectrically-active junctions between the iron-nickel ends of the reed switch portion and the copper circuit wires are made in close proximity to one another within an isothermal enclosure and, along with the ends of the reed switch portion, are substantially thermally shorted together so as to reduce the temperature differential that can be created between the thermoelectrically-active junctions. Fine copper circuit wires having a relatively high thermal impedance compared to the ends of the reed switch portion and the larger copper wires of the external circuit are used to connect the ends of the reed switch portion and the end terminals of the reed relay so as to minimize the thermally unbalancing effects of external temperature gradients.

Other and incidental objects of this invention will become apparent from a reading of this specification and an inspection of the accompanying drawing.

Description of the drawing FIGURE 1 is a perspective view of a reed relay according to the preferred embodiment of this invention.

FIGURE 2 is a sectional view taken along the line A-A in FIGURE 1.

FIGURE 3 is a sectional view taken along the line BB in FIGURE 1.

FIGURE 4 is a sectional view taken along the lines C-C in FIGURES 1, 2, and 3.

Description of the preferred embodiment Referring now to the drawing, and particularly to FIG- URE 4, there is shown a continuous cylindrical bobbin 10 made of copper, aluminum, or some other metal possessing a high rate of thermal conductivity. A pair, for example, of reed switches 12 are supported side-by-side within a central portion of a cylindrical bore 14 that extends through the bobbin 10 along its longitudinal axis, and a reed switch drive coil 16 is wound on the outer surface of a cylindrically-recessed central portion of the bobbin. The reed switches 12 should be selected for high sensitivity so that less power need be dissipated in the drive coil 16. They may comprise, for example, IBM number 765972 reed switches. The drive coil 16 should be wound on the bobbin 10 very evenly and symmetrically so that one end of the bobbin will not receive heat at a greater rate than the other. The drive coil 16 may comprise a very fine copper wire of a size, voltage rating, and number of turns that depend on the particular reed switches 12 that are used. This copper wire is insulated to prevent the turns of the drive coil 16 from shorting together.

The bobbin 10 serves as a continuous electrostatic shield between the drive coil 16 and the reed switches 12. In addition, it is made with a sufficiently large heat capacity to efiiciently remove heat from the drive coil 16 so that this heat cannot contribute to a temperature differential between the ends 18 of the reed switches 12. This substantially reduces the thermal EMF of the reed relay. The heat transferred from the drive coil 16 to the bobbin 10 may easily be coupled to the surrounding environment by providing air circulation around the bobbin 10 or by coupling the bobbin to a heat sink such as the chassis of an instrument.

The reed switches 12 are supported within the bobbin 10 by a pair of discs 20- made of a material having a high insulation resistance so as to minimize leakage current from the reed switches and having a low dielectric constant so as to minimize the capacitance between the reed switches and between the bobbin and the reed switches. For example, the discs 20 may be made of beryllium oxide, Teflon, or beryllium oxide coated with a thin layer of Teflon. The discs 20 are press-fitted at the opposite ends of the bobbin 10 into the bore 14 until they abut upon shoulders 22 formed near the ends of the bobbin by a reduction in the diameter of the central portion of the bore 14. Two hollow, cylindrical copper slugs 26 are press-fitted into a first pair of spaced holes bored through one of the discs 20, and two similar copper slugs 26 are press-fitted into a second pair of spaced holes bored through the other disc 20 in substantial alignment with the first pair of holes. Each reed switch 12 is supported within the central portion of the bore 14 by inserting one of its ends 18 into one of the copper slugs 26 of one of the discs 20 through a hole in the back of that copper slug and the other of its ends 18 into a corresponding one of the copper slugs 26 of the other disc 20 through a hole in the back of that copper slug and by soldering the ends 18 in place so that the hollow copper slugs 26 are filled with solder 28. Cylindrical glass spacers 30 are supported on the reed switch ends 18 so as to position each reed switch 12 between the discs 20 and within the drive coil 16. The copper slugs 26 provide connections of equal thermal resistance and contact area to the ends 18 of the reed switches 12 and reduce the length of the thermal path between these ends and the bobbin through the less thermally-conductive discs 20. They also tend to minimize small thermal disturbances induced by convective air currents on the fine copper wires 32 to which they are coupled.

Although the reed switches 12 may be used as independent switching units, for purposes of illustrating the preferred embodiment of this invention they are shown serially connected for use as a single switching unit so that the thermal EMFs they produce tend to cancel one another. The reed switches 12 are serially connected, as best shown in the rightend view of FIGURE 2, by bringing one end 18 of one of the reed switches 12 out through the copper slug 26 to which it is soldered and inserting it back into the adjacent copper slug 26 where it is additionally soldered or welded to a corresponding end 18 of the adjacent reed switch 12.

As shown in the left-end and sectional views of FIG- URES 3 and 4, respectively, the two remaining reed switch ends 18 are soldered at the copper slugs 26 in which they are mounted to a pair of very fine copper wires 32 for connecting these ends to a pair of electrically-conductive end terminals 34 of the reed relay. The thermoelectricallyactive junctions between the fine copper wires 32 and the iron-nickel reed switch ends 18 are therefore placed in close proximity to one another so as to reduce the temperature dilferential that can be created between these junctions by ambient temperature gradients. This temperature differential is further reduced because these junctions, as well as the reed switch ends 18, are all substantially thermally shorted together by a substantially thermally-conductive path extending through the solder-filled copper slugs 26, the discs 20', and the bobbin 10.

The bobbin 10 may be mounted within a copper or aluminum case 36 so that the end portions of the bobbin are thermally coupled to the case. This enhances the thermal short connecting the thermoelectrically-active junctions (as well as the reed switch ends 18) together. In addition, the case 36 serves as an electrostatic shield and as an isothermal enclosure for the bobbin assembly. In its role as an isothermal enclosure the case 36 stagnates the air around the bobbin assembly so as to minimize the temperature gradients caused by convective air currents and thereby further reduce the temperature diiferential that can be created across the thermoelectrically-active. junctions of the reed relay. The case 36 includes two side pieces 38 that are conformed to hold the bobbin 10 and that are fastened together in contact with the end portions of the bobbin by clamping screws 40, which pass through notches 42 in the end portions of the bobbin so as to properly align the bobbin with the side pieces 38. These side pieces 38 provide suitable openings so that the input terminals 44 of the drive coil 16 may be brought out to a source of drive voltage for the reed relay. The electricallyconductive end terminals 34 of the reed relay are pressfitted into holes bored near the left end of the case 36 through beveled portions '46 of the side pieces 38. They are electrically insulated from the conductive case 36 by Teflon sleeves 48. The fine copper wires 32 soldered to the reed switch ends 18 at the left disc 20 are several inches in length. They are supported by winding them a few times around electrically and thermally-insulated support posts 50, which are press-fitted into holes bored through this disc 20 beneath its copper slugs 26, and are connected to the end terminals 34 of the reed relay. Since the fine copper wires 32 have a relatively high thermal impedance compared to the reed switch ends 18 and the larger copper Wires of the external circuit, they tend to minimize the thermally unbalancing effects of heat propagated along the copper wires of the external circuit. The case 36 also includes a pair of thermally-conductive end plates 52 that are attached by screws 54 to the opposite ends of the side pieces 38 so as to isothermally enclose the thermoelectrically-active junctions and the ends of the reed switches 12.

The above-described features of this reed relay substantially reduce the thermally unbalancing effects of heat dissipation in the drive coil 16 and of temperature gradients both internal and external to the reed relay so that the thermal EMF generated during steady state operation of the reed relay may be reduced to less than one microvolt. For example, such a reed relay has been built employing two serially-connected IBM number 765972 reed switches 12 and a drive coil 16 comprised of about six thousand turns of #43 copper wire insulated with single formvar insulation. The dimensions of the Teflon discs 20 were made such that a capacitance of less than two picofarads and a resistance of more than 10 ohms at ninetyfive percent of the rated coil heating were obtained between the reed switch leads 24 and the case 36. For a temperature rise in the drive coil 16 of approximately one to two degrees centigrade the thermal EMF, or ofiset, was only about two-tenths of a microvolt. Other reed relays employing only one or as many as three reed switches 12 have been built in accordance with the features of this invention so as to reduce the thermal EMFs produced during steady state operation of the relay to less than one microvolt. The size of the bobbin 10 and the number of copper slugs 26 and insulated posts 50 that are mounted in the discs 20 must accordingly be varied to accommodate the number of reed switches 12 that are used.

We claim:

1. A reed relay comprising: a hollow, thermally-conductive body; a pair of reed switches supported within said body, each of said switches including first and second reeds sealed within an electrically-insulating enclosure and first and second reed ends extending outside this enclosure, the first and second reeds of each of said switches having an engaged and an unengaged position and being normally biased to one of said positions; and a reed switch drive coil supported on an outer surface of said body in position for biasing the first and second reeds of each of said switches to the other of said positions in response to energization of the drive coil; wherein said reed relay is improved by said reed switches being supported within said body in side-by-side relationship with their first reed ends positioned adjacent to one another for connection to a pair of lead wires with which the first reed ends form thermoelectrically-active junctions and with their second reed ends positioned adjacent to one another and serially connected so that the thermal EMFs produced by said switches tend to cancel one another thereby reducing the thermal EMF produced by the reed relay.

2. A reed relay as in claim 1 further improved by including a substantially thermally-conductive and electrically-insulating path for substantially thermally shorting the thermoelectrically-active. junctions and the reed ends of said switches together to further reduce the thermal EMF produced by the reed relay.

3. A reed relay as in claim 2 wherein said path comprises: said thermally conductive body; and a pair of substantially thermally-conductive and electrically-insulating support members fixedly positioned adjacent to opposite end portions of said body for supporting said switches within said body, said support members being thermally coupled to the opposite end portions of said body, to said thermoelectrically-active junctions and to the reed ends of said switches.

4. A reed relay as in claim 3 wherein: said body is a bobbin having a cylindrically-recessed central portion along its outer surface around the region of said switches for supporting said drive coil; and said support members are discs comprising at least one of beryllium oxide and Teflon.

5. A reed relay as in claim 4 including an isothermal enclosure within which said bobbin is supported and to which said bobbin is thermally coupled, said enclosure thereby serving to still further reduce the thermal EMF produced by the reed relay.

6. A reed relay comprising: a hollow, thermally-conductive body; at least one reed switch supported within said body and including first and second reeds sealed Within an electrically-insulating enclosure and first and second reed ends extending outside this enclosure, said first and second reed ends having an engaged and an unengaged position and being normally biased to one of said positions; and a reed switch drive coil supported on an outer surface of said body around the region of said switch for biasing said first and second reeds to the other of said positions in response to energization of the drive coil; wherein said reed relay is improved by providing a substantially thermally-conductive and electrically-insulating path for substantially thermally shorting the reed ends of said switch together to reduce the thermal EMF produced by the reed relay, said path including said thermallyconductive body and a pair of substantially thermallyconductive and electrically-insulating support members fixedly positioned adjacent to opposite end portions of said body for supporting said reed switch within said body, said support members comprising at least one of beryllium oxide and Teflon and being thermally coupled to said body and to the reed ends of said switch.

7. A reed relay as in claim 6 wherein: said body is a bobbin having a cylindrically-recessed central portion along its outer surface around the region of said switch for supporting said drive coil; and said support members are discs.

8. A reed relay as in claim 7 wherein: each of said support discs includes a thermallyand electrically-conductive slug; said switch is fixedly supported within said bobbin between said support discs With each of its reed ends attached to a diiferent one of said slugs; and said reed relay further comprises means including a pair of fine wires for electrically coupling the first and second reed ends of said switch to external circuit wires, said fine wires having a higher thermal impedance than the reed ends or the external circuit wires to impede the transfer of heat from the external circuit wires to the junctions formed between the reed ends and they fine wires thereby further reducing the thermal EMF produced by the reed relay.

9. A reed relay as in claim 8 including: an isothermal enclosure within which said bobbin is supported and to which said bobbin is thermally coupled to still further reduce the thermal EMF produced by the reed relay; and first and second electrically-conductive end terminals to which said fine wires are connected, said end terminals being insulated from but supported by said isothermal enclosure.

References Cited UNITED STATES PATENTS 3,005,069 10/1961 Sippach et al. 335-163 X 3,201,540 8/1965 McNamara 335-151 3,305,803 2/1967 Highly et a1. 335-97 3,308,408 3/1967 Plice 33515l 3,320,559 5/1967 Morrison 335151 BERNARD A. GILHEANY, Primary Examiner R. N. ENVALL, JR., Assistant Examiner US. Cl. X.R. 3 35l5 4 

